The rate constants, k(CR), of o-Ps into p-Ps conversion reactions promoted by paramagnetic 3d complexes are linearly correlated with the delocalization, beta, of 3d electrons towards the periphery of the compounds. Therefore, providing known beta, k(CR) may be estimated and vice versa. The beta value may be calculated by means of the following empirical equation: beta = 1-th, where t and h are constants characteristic of the ion and ligands, respectively. By measuring the k(CR)s of six Fe-II complexes and by using the correlations mentioned above, the 0 and then the t(Fe-II) values, not reported in literature, were determined. A relationship between t and the oxidation number of the metal ions is discussed.